[linux-lvm] LVM performance anomalies

I have some rather odd performance anomalies when using direct i/o
towards lvm logical volumes.

I have a setup with 10 external scsi disks attached to the external
scsi port of a DELL PERC 4DC RAID controller. Each disk is configured
as its own logical disk, hence not using raid functionality in the
adapter .

* The disks is pvcreated with "pvcreate /dev/sd[d-m]" .
* The vg is created with vgcreate vg_perc /dev/sd[d-m] .

Suddenly, in the middle of one the runs, the throughput drops to
128-256kB/s pr LV. If I stop the job and start a job reading directly
from the underlying disks I get 39MB/s aggregated . If I start the
job against LVs I still get maximum 512kB/s .

completion latency and bandwidth log plots (from fio) of a job with
poor throughput is seen here: